Method and apparatus for plasma treatment of gas

ABSTRACT

A method and an apparatus for plasma treatment of gas, particularly for transformation, passivation and stabilization of polluting and toxic admixtures is disclosed. A flowing gas ( 1 ) to be treated is passed through a hollow cathode ( 2 ) provided with a counter electrode ( 5 ) and a hollow cathode discharge plasma ( 3 ) is generated by a generator ( 4 ) connected to the hollow cathode ( 2 ). The flowing gas ( 1 ) undergoes interactions with oscillating electrons ( 6 ) in the hollow cathode discharge plasma being generated in the gas. The interaction of the hollow cathode discharge plasma with inner walls ( 8 ) of the hollow cathode is controlled and the inner walls should have a temperature below its melting point, whereby the inner walls can provide a catalytic effect. The inner walls may also release wall species ( 9 ) promoting plasma-chemical reactions in the hollow cathode discharge plasma. The flowing gas is then exhausted as a transformed gas ( 10 ) after being treated in the hollow cathode discharge plasma. A gas to be treated may also be mixed with an auxiliary gas ( 7 ) for obtaining a suitable mixing of gas, whereby the auxiliary gas intensifies an ultraviolet radiation and/or plasma-chemical reactions.

RELATED APPLICATION

[0001] This application is a division of co-pending application Ser. No.09/303,574, filed on May 3, 1999, the entire contents of which arehereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a method and an apparatus forplasma treatment of gas, particularly for transformation, passivationand stabilization of polluting and toxic admixtures.

BACKGROUND

[0003] The treatment of different solid, liquid and gas wastes andhazardous residues represents a very important environmental technology,particularly treatment of industrial flue gases and exhaust gases fromengines, due to their pollution of the air by NO_(x), SO_(x), soot,hydrocarbons, volatile organic compounds, etc. The most common methodsare based on chemical reactions, pyrolytic combustion and differentcatalyst filters. Chemical filters are often used for treatment of fluegas from industrial and energy production. They can treat largethroughputs of gases, but their substantial disadvantage is formation ofbyproducts (e.g., NH₃, CaO₂, etc.). Catalysts are regularly used forinstance in car exhaust after-treatment. Typical catalyst materials arebased on Pt or different metal oxides e.g., V₂O₅, WO₃, TiO₂, ZnO, etc.Despite their simplicity the catalyst filters often exhibit limitedefficiency, particularly in Diesel engines, and strong temperaturedependence.

[0004] Contrary to the conventional chemical and pyrolyticaltechnologies, the gas discharge plasma treatment often leads toapparently more efficient transformation reactions with little or noundesirable byproducts. This is given by the possibilities either toachieve extremely high gas temperatures in the plasma, or to generatevery reactive species through dissociation, activation or ionization ofgaseous and volatile components in the plasma. Pyrolytic effect ofplasma can be achieved for instance in atmospheric pressure arc torchesat high powers generating large current densities in the ionized gas.These plasmas are called equilibrium or thermal plasmas and they arecharacterized by high collision frequencies, which equalize energy ofall particles present. The energies (temperature) of electrons and ionsare practically the same and they approach the energy of neutral speciesin the partially ionized plasma. The gas temperature is high, up toseveral thousands degrees centigrades, hence the term “thermal” plasma.The thermal plasma is very suitable for irreversible thermal treatmentse.g., for combustion of solid and liquid wastes, or in plasmametallurgy, etc. However, due to heating of all species evenly theenergy consumption in thermal plasmas is typically high. At equilibriumconditions the gaseous products of plasma chemical reactions are oftenunstable due to almost equivalent probability of reverse chemicalreactions. The efficiency of thermal plasmas for plasma chemicaltreatment of gases is therefore low in comparison with so callednon-equilibrium plasmas. Non-equilibrium plasma can be simply generatedat reduced gas pressure. Then the frequency is lower and due todifferent electron and ion masses (the electron mass is 9.11×10⁻³¹ kg,the proton mass is 1.67×10⁻²⁷ kg) the electrons can acquire much higherkinetic energy than ions. When the plasma is generated by a very highfrequency electromagnetic field, the power is acquired mainly by mobileelectrons, while heavy ions are not able to even follow changes of thefield and move only due to their thermal energies similar to the rest ofthe neutral gas. This leads to the plasma in which the chemicalreactions are very effective while the bulk of gas remains relativelycold. The non-equilibrium plasma is therefore often noted as “cold”plasma. Interactions of high-energy electrons with gas can produceextremely reactive atoms and radicals, which are able to generatesubsequent chemical reactions not available at normal conditions. A veryhigh plasma chemical activity of such plasmas can be utilized indifferent applications (see for instance a pioneering work of F. K.McTaggart: “Plasma Chemistry in Electrical Discharges”, Elsevier,Amsterdam, 1967). However, due to necessity of pumping systems, thereduced and low-pressure non-equilibrium plasmas are not utilized inindustrial scale waste treatments.

[0005] An emerging technology in this field is non-equilibrium plasma ofatmospheric pressure. The degree of energy non-equilibrium is somewhatlower than in low-pressure plasmas and strongly depends on thearrangement of individual reactors. However, the absence of pumpssimplifies all systems substantially and in principle allows theirimmediate application for large gas throughputs. On the other hand theefficiency of known systems is still not high enough for theirutilization in an industrial scale. The non-equilibrium conditions atatmospheric pressure can be achieved in several ways. The most directway is an injection of an electron beam into the gas. The high powerelectron beam interacts with the gas and generates non-equilibriumplasma along its penetration depth. A serious disadvantage of the methodis that it acts only in a limited space and that the penetration depthat atmospheric pressure is rather short. Moreover the electron gunsystem is quite complicated and expensive and it does not showsatisfactory high energy efficiency. The more common way of generationof non-equilibrium plasma for gas treatment is a high voltage breakdownof the gas in form of many filamentary current paths—plasma streamers. Atypical representative is corona discharge between sharp edged or sharptipped electrode (cathode or anode) and the grounded counter electrode.At a high frequency generation (orders of 1 kHz up to more than 1 MHz)and high voltage (10-30 kV), the counter-electrode may be covered by adielectric wall (barrier) and then the system works with a barrier (also“silent”) discharge.

[0006] Another very sophisticated reactor is composed from an axialsystem of at least one pair (typically three pairs) of knife sharpelectrodes facing each other by sharp edges and connected to the highvoltage generator, see for instance French patent No. 2639172 (1988) toH. Lesueur et al. Arc streamers between related electrodes are glidingover the sharp edges between opposite electrodes and also around theaxial system of electrodes following the phase movement in a 3-phasegenerator.

[0007] The generation of local current streamers in all systemsmentioned above provides local non-equilibrium plasmas when the rest ofthe gas remains “cold”. A great advantage found in coronas and barrierdischarges is a pulsed generation, see U.S. Pat. No. 5,603,893 (1997) toM. Gunderson et al.. The high power pulse allows quick pumping of thepower into the streamers causing strong non-equilibrium plasma usuallyat the beginning of the pulse with the relaxation into the equilibriumconditions depending on both the pulse shape and the duty cycle.Although all these systems in both stationary and pulsed regimes arevery advanced, the region where the gas interacts with streamers is notdense enough or of sufficient volume (bulk) for treatment of all the gaspassing the reactor zone. Generation of bulk atmospheric pressurenon-equilibrium plasma is possible by microwave power. This type ofgeneration is based on very high frequency (typically 2.45 GHz and more)connected with the pumping of power directly into electrons. Althoughthe advantage of microwave systems is a denser plasma volume than in thecase of streamers, substantial disadvantages are low efficiency andshort lifetime of microwave generators and limited plasma dimensions(related to the size of the wave-guide). Therefore, in spite of theirdiscrete streamer character the most serious candidates considered forindustrial atmospheric pressure plasma treatment of gas are still thepulsed corona, pulsed barrier discharge and the gliding arc.

[0008] Non-equilibrium plasma with very high degree of activation can begenerated at reduced pressures by hollow cathodes. Since their discoveryby F. Paschen in 1916, the hollow cathodes have been used for quite along time as the sources of intense light for atomic spectroscopy.Experiments of Little and von Engel in 1954 revealed clearly theprinciple of an exceptionally high plasma density and activation inhollow cathodes through so called “hollow cathode effect”. This effectis based on a special geometry in the cathode, where the opposite wallshave the same electric potential with respect to a common anode. In adirect current (DC) arrangement of the diode gas discharge the cathodewall is covered by cathode dark space in which electrons emitted fromthe cathode surface are accelerated towards the anode. In the suitable“hollow geometry” when the cathode fall regions of opposite cathodesurfaces are close to each other, the emitted electrons can meet theopposite region with the equal opposite electric field of the cathodefall region. Electrons are therefore repelled back and undergooscillations called a “pendulum electron motion”. This kind of motionleads to confinement of electrons and intensifies their interactionswith the gas present in the hollow cathode, which promotes dramaticallyoverall efficiency of the ionization and subsequent effects, terminatingin a very high density active plasma. Hollow cathodes are capable ofproduction of electron beams having energies comparable to the cathodefall potential and, moreover, their extraordinary abilities are alreadyknown also for generation of different plasma chemical reactions ingases for applications mainly in surface processing. Moreover, due tonon-Maxwellian energy distributions and existence of high energyelectron populations the hollow cathode discharge emits an intenseradiation in UV (≦300 nm) and VUV (≦200 nm) regions capable to breakdownmost chemical bonds and to induce different photochemical reactions.

[0009] Compared to the DC generation of the hollow cathode, analternating current (AC) and particularly a radio frequency (RF)generation provides a number of advantages. In this case the mostpositive body in the system is the gas discharge plasma outside thecathode. This plasma can substitute a “virtual anode” and is naturallyflexible with respect to any cathode geometry. As a consequence, thehollow cathode behaves as a unipolar discharge (see e.g. L. Bárdos etal., Thin Solid Films 1987, or recent reviews by L. Bárdos et al., Surf.Coat. Technol. 1996 and 1997). The AC generation has also an importantthermal stabilizing effect.

[0010] To satisfy conditions for the hollow cathode effect the distancebetween the opposite walls in the cathode must be in suitable relationwith the thickness of the cathode fall, or the space charge sheath inthe RF case, to enable the electron exchange. One of the most importantparameters is the gas pressure, which affects both the thickness of thecathode regions and the electron recombination by collisions. Thereforethe hollow cathodes are operated typically at reduced pressures, belowthe order of 10 Torr. Very recently the cylindrical closed end DCmolybdenum hollow cathodes with diameters below 0.1 mm were reported towork at air pressures of 350 Torr ≈50 kPa (K. H. Schoenbach et al.,Appl. Phys. Lett. 1996). Similar DC cathodes with diameters of 0.2-0.4mm and 0.5-5 mm in depth were reported to sustain the nitrogen dischargeeven beyond the atmospheric pressure (>750 Torr ≈100 kPa), see J. W.Frame et al., Appl. Phys. Lett, 1997. Atmospheric pressure xenondischarges were generated in a 0.1 mm diameter dc hollow cathode by A.Al-Habachi and K. H. Schoenbach (Appl. Phys. Lett. 1998). These DChollow cathode discharges were able to produce intense UV and VUVexcimer emissions. Arrays of the closed end micro-hollow cathodes havebeen used in UV lamp applications, see U.S. Pat. No. 5,686,789 (1995) toK. H. Schoenbach et al.

[0011] No works, arrangements or results have yet been found publishedregarding the utilization of hollow cathodes for treatment of a flowinggas or for passivation of polluting and toxic gas mixtures neither atatmospheric pressure nor at reduced pressures. Moreover there are noworks known yet reporting on AC generated hollow cathodes or relatedreactors for this purpose at atmospheric pressure.

SUMMARY

[0012] An object of present invention is therefore to overcome thedrawbacks of the above described prior art techniques and to provide amethod and an apparatus for plasma treatment of gas, particularly fortransformation, passivation and stabilization of polluting and toxicadmixtures.

[0013] In a first aspect according to the present invention, a methodfor plasma treatment of gas comprises steps of flowing a gas to betreated through a hollow cathode with a hollow cathode discharge plasmabeing generated by a generator coupled with said hollow cathode and witha counter electrode. The gas undergoes interactions with oscillatingelectrons in the hollow cathode discharge plasma generated in the gas orin a mixture of the gas and an auxiliary gas, in which the auxiliary gasintensifies an ultraviolet radiation and/or plasma-chemical reactions.The hollow cathode discharge plasma also interacts with the inner wallsof the hollow cathode having a temperature below its melting point, theinner walls may have a catalytic effect and may also release wallspecies promoting plasma-chemical reactions in the hollow cathodedischarge plasma. After being treated in the hollow cathode dischargeplasma the gas will be flowing out as a transformed gas.

[0014] In a second aspect according to the present invention anapparatus is disclosed for plasma treatment of gas according to themethod of the present invention, particularly for transformation,passivation and stabilization of polluting and toxic admixtures. Theapparatus comprises generator means, particle filter means, solid statecatalyst means and the means for heating and/or cooling. The apparatusconsists of at least one hollow cathode connected with the means forheating and/or cooling, whereby the cathode has a length, which islonger than a distance between opposite walls and is disposed forgeneration of a hollow cathode discharge plasma by means of thegenerator coupled with the hollow cathode and a counter electrode. Thecounter electrode is disposed as an inlet of a gas to be treated in thehollow cathode discharge plasma generated in the gas or in a mixture ofthe gas and an auxiliary gas, whereby the particle filter is connectedwith the means disposed in the counter electrode for filtering of thegas to be treated. The filter is installed upstream the gas before thehollow cathode and a quenching electrode connected with the means forheating and/or cooling is a transformed gas after the hollow cathode andthe solid state catalyst for additional treatment of the transformedgas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The objects, features and advantages of the present invention asindicated above will become apparent from the description of theinvention given in conjunction with the appended drawings, wherein:

[0016]FIG. 1 is a schematic representation of the method for plasmatreatment of gas, particularly for transformation, passivation andstabilization of polluting and toxic admixtures according to the presentinvention;

[0017]FIG. 2 is a schematic view of an embodiment of an apparatus forplasma treatment of gas, particularly for transformation, passivationand stabilization of polluting and toxic admixtures according to themethod described in the present invention;

[0018]FIG. 3 demonstrates different geometries of integrated systems ofhollow cathodes for treatment of gas in apparatus according to thepresent invention;

[0019]FIG. 4 shows in a schematic view a cross section of an integratedsystem of cylindrical hollow cathodes aligned with a complementarysystem forming cathode terminations with sharp edges for enablinginitiation and maintenance of the hollow cathode discharge plasma;

[0020]FIG. 5 shows in a schematic view a cross section of an integratedsystem of hollow cathodes with conical forms aligned with acomplementary system having a set of sharp cones centered in the hollowcathodes for enabling initiation and maintenance of the hollow cathodedischarge plasma;

[0021]FIG. 6 shows in a schematic view a cross section of an integratedsystem of hollow cathodes with opposite system of sharp tips arranged ata quenching electrode for enabling initiation and maintenance of thehollow cathode discharge plasma;

[0022]FIG. 7 shows in a schematic view an integrated system of hollowcathodes with a particle filter arranged as a counter electrode; and

[0023]FIG. 8 finally shows in a schematic view an apparatus according tothe present invention where a transformed gas is used for subsequenttreatment of surfaces.

DETAILED DESCRIPTION

[0024] Referring to FIG. 1, the method according to the presentinvention will be described. A gas 1 to be treated is flowing through ahollow cathode 2 with a hollow cathode discharge plasma 3 generated by agenerator 4 connected to the hollow cathode and with a counter electrode5. The gas 1 undergoes interactions with oscillating electrons 6 in thehollow cathode discharge plasma. The plasma is generated in the incominggas to be treated or in a mixture of this gas with an auxiliary gas 7.The auxiliary gas could be admitted for intensifying an ultravioletradiation and/or plasma-chemical reactions. Typical example may be anexcimer (excited dimer) gas, which forms a slightly bound excitedmolecular state of complexes that do not possess a stable ground state.Besides halogen dimers, for example iodine, I₂, or chlorine, Cl₂, alsonoble gases like xenon, Xe, or argon, Ar, or their mixtures can be usedto form excimer complexes in plasma state (see B. Gellert and U.Kogelschatz, Appl. Phys. B 52, 1991). The excimer gas causes excimerradiation typically in UV or VUV regions originated inside the hollowcathode discharge plasma. This radiation may be capable to destroy mostchemical bonds and the auxiliary gas can therefore play the role of an“in-situ” gas catalyst in the hollow cathode discharge plasma, duringthe treatment of the gas. The hollow cathode discharge plasma interactswith the inner walls 8 of the hollow cathode, which may become hot dueto ion bombardment and must be kept below its melting temperature. Thesurface of inner walls may have a catalytic effect if the cathodematerial or a wall coating is properly selected. Typical catalyticmaterial is platinum, Pt, but also other metals and oxides may beselected, depending on expected catalytic effect. Due to ion bombardmentthe inner walls of the hollow cathode may also release wall species 9promoting plasma-chemical reactions in the hollow cathode dischargeplasma. This type of catalysis can be very important in an overallefficiency of the gas treatment in the hollow cathode discharge plasmaaccording to the present invention. After being treated in the hollowcathode discharge plasma the gas is flowing out as a transformed gas 10.

[0025] Referring to FIG. 2, an embodiment of the apparatus for plasmatreatment of gas according to the method described in the presentinvention will be described. At least one hollow cathode 2 is used andconnected to the means 20 for heating and/or cooling and has a length 11longer than a distance 12 between opposite walls 8. A suitable ratiobetween this length 11 and the distance between the walls 8 (width)should exceed a factor of two. The appropriate width of the cathodecould preferably be less than 1 mm. Walls 8 could either be composed ofor coated by a catalyst material. The cathode is disposed for generationof a hollow cathode discharge plasma 3. The plasma is generated by agenerator 4 connected to the hollow cathode and a counter electrode 5,whereby the counter electrode could favorably serve also as an inlet ofa gas 1 to be treated. Although both DC and AC generators can be used,for the purpose of this invention an AC generator should provide powerof high frequency. Higher frequencies, for instance radio frequencies upto 100 MHz, are favorable for generation of non-equilibrium plasma, inwhich only electrons are capable to follow changes of the field andabsorb its energy. Pulsed power is also suitable for generation ofnon-equilibrium chemically active plasma. The hollow cathode dischargeplasma is generated either in the gas to be treated or in a mixture ofthis gas and an auxiliary gas 7. The particle filter 15 can be installedin the counter electrode and is disposed for mechanical filtering of thegas 1. The particle filter is connected with the means 16 for heatingand/or cooling. Preheating of the gas by the filter can favorablyenhance chemical reactions during the gas treatment in the hollowcathode discharge plasma. A quenching electrode 13 is installeddownstream the gas after the hollow cathode. The quenching electrode isdisposed to slow down chemical reactions in the gas flowing aftertreatment in the hollow cathode discharge plasma and to stabilize a partof resulting products. The temperature of the quenching electrode isvery important and the electrode is therefore connected to the means 17for heating and/or cooling. The solid state catalyst 14 is disposed foradditional treatment of the transformed gas flowing from the hollowcathode discharge plasma. A parameter detector system 18 may beinstalled for detection of parameters of the hollow cathode dischargeplasma, for instance through an optical emission. The detector can beused through a suitable feedback electronics circuitry for control ofthe generator 4. Moreover an outlet parameter detector system 19installed in the transformed gas 10 can be used in similar way forcontrol of the generator 4. Both detectors can be used in an advancedarrangement also for control of the optimal temperature of the particlefilter, the hollow cathode and the quenching electrode, respectively.The treatment of the gas 1 in the hollow cathode discharge plasma 3 maybe enhanced by a magnetic field generated by magnets 21.

[0026] By means of FIGS. 3 to 8 a number of embodiments for the presentapparatus will be discussed. Referring to FIG. 3 a first example will bedescribed, which is related to the method and the apparatus according tothe present invention shown in FIG. 1 and FIG. 2. The schematic figureshows different suitable geometries of integrated systems 22 of hollowcathodes 2 for treatment of gas. FIG. 3(a) shows cylindrical system withan array of cylindrical hollow cathodes. FIG. 3(b) shows a rectangularsystem of rectangular hollow cathodes. Note that such a geometry may beassembled as a multi-layer system of metal grids. FIG. 3(c) shows thecylindrical system with an array of parallel rectangular hollowcathodes. FIG. 3(d) shows rectangular system with parallel rectangularhollow cathodes. FIG. 3(e) is a cylindrical system with concentrichollow cathodes and finally FIG. 3(f) is a cylindrical system with aninserted structure made from planar metal foil and wrapped metal foilplaced on top of each other and rolled into cylinder shape.

[0027] Referring to FIG. 4 an embodiment will be described, which isrelated to the method and the apparatus according to the presentinvention. A schematic figure shows a cross section of an integratedsystem 22 of cylindrical hollow cathodes aligned with a complementarysystem 23 forming cathode terminations with sharp edges. Both systemsmay be fabricated and aligned by standard methods of micro-mechanics.The sharp edges enhance generation of the discharge at high gaspressures, for instance by forming of a plasma torch, which enablesinitiation and maintenance of the hollow cathode discharge plasma.

[0028] Referring to Fig.. 5, another embodiment will be described, whichis related to the method and the apparatus according to the presentinvention. The schematic figure shows a cross section of an integratedsystem 22 of hollow cathodes 2 with conical forms aligned with acomplementary system 23 with a set of sharp cones centered in the hollowcathodes 2. Both systems may be fabricated and aligned by standardmethods of micro-mechanics. The sharp cones produce high intensity ofelectric field and enable initiation and maintenance of the hollowcathode discharge plasma.

[0029] Referring to FIG. 6, still a further embodiment will bedescribed, which is related to the method and the apparatus according tothe present invention. The schematic figure shows a cross section of anintegrated system 22 of hollow cathodes 2 with an opposite system ofsharp tips 25 arranged at the quenching electrode 13. The quenchingelectrode in this example is arranged as a counter electrode connectedto a counter pole of the generator 4. A high voltage applied betweeneach tip 25 and the edges of an opposite facing hollow cathode 2 causesgas breakdown and can generate filamentary discharges of a corona type.This discharge enables initiation and maintenance of the hollow cathodedischarge plasma. Moreover the system can work as a hybrid plasma systemof hollow cathode and corona discharges for treatment of the gas. Thetransformed gas 10 will flow through a system of holes 26 provided inthe quenching electrode 13.

[0030] Referring to FIG. 7, still a further embodiment of the inventionwill be described, which is related to the method and the apparatusaccording to the present invention. The schematic figure shows a simpleschematic view of an integrated system 22 of hollow cathodes 2 with theparticle filter 15 arranged as a counter electrode and connected to thegenerator 4. It is to be noted that besides the cartridge 5, quenchingelectrode 13 (see FIG. 6) and the particle filter, the role of counterelectrode may also have a solid state catalyst 14 (shown in FIG. 2), orall respective components together, or their combinations.

[0031] Finally referring to FIG. 8 an example of utilization of themethod and the apparatus according to the present invention fortreatment of surfaces will be described. The schematic figure shows asimple schematic view of an arrangement similar to that in FIG. 7, wherethe transformed gas 10 is a plasma activated gas suitable for subsequenttreatment of a surface 27 facing the integrated system 22 of hollowcathodes 2 downstream the gas flow. In this case the quenching electrode13 and the solid state catalyst (shown in FIG. 2) may be omitted.Surface 27 may be of solid or liquid state. However a treatment of“gaseous surfaces” (i.e. other gases) by the transformed gas 10 from theapparatus according to the present invention may be considered, too.

[0032] The inventive method and apparatus for plasma treatment of gas,particularly for transformation, passivation and stabilization ofpolluting and toxic admixtures is advantageous for being applied for thecleaning of large amounts of flue gas from industrial production, forinstance, when arranged in multi-modular systems of filters. Singlemodules with integrated systems of hollow cathodes made bymicro-mechanic technology are suitable for plasma exhaustafter-treatment of engines like Diesel motors, turbines, etc. The gastreatment according to the invention can also be used for production ofactivated gas suitable for subsequent treatment of stationary or movingsurfaces. Plasma filters according to the invention can also serve infinal or integrated cleaning stages in conventional cleaning andfiltering systems.

[0033] It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the spirit and scope thereof, which is defined by theappended claims.

What is claimed is:
 1. A method for plasma treatment of gas,particularly for transformation, passivation and stabilization ofpolluting and toxic admixtures comprising the steps of: inserting aflowing gas to be treated through a hollow cathode provided with acounter electrode; generating a hollow cathode discharge plasma by agenerator connected to said hollow cathode, whereby said flowing gaswill undergo interactions with oscillating electrons in said hollowcathode discharge plasma being generated in said gas; controlling theinteraction of said hollow cathode discharge plasma with inner walls ofsaid hollow cathode, said inner walls having a temperature below theirmelting point, whereby said inner walls can provide a catalytic effectand may also release wall species promoting plasma-chemical reactions insaid hollow cathode discharge plasma; exhausting said flowing gas as atransformed gas after being treated in said hollow cathode dischargeplasma.
 2. The method of claim 1 , comprising the further step of mixingthe flowing gas to be treated with an auxiliary gas for obtaining asuitable mixing of gas, whereby said auxiliary gas intensifiesultraviolet radiation and/or plasma-chemical reactions.
 3. An apparatusfor plasma treatment of gas, particularly for transformation,passivation and stabilization of polluting and toxic admixtures,comprising: means for inserting a flowing gas to be treated through ahollow cathode provided with a counterelectrode; means for generating ahollow cathode discharge plasma by a generator connected to said hollowcathode; whereby said flowing gas will undergo interactions withoscillating electrons in said hollow cathode discharge plasma beinggenerated in said gas; means for controlling the interaction of saidhollow cathode discharge plasma with inner walls of said hollow cathode,said hollow walls having a temperature below their melting point,whereby said inner walls can provide a catalytic effect and may alsorelease wall species promoting plasma-chemical reactions in said hollowcathode discharge plasma; and means exhausting said flow gas as atransformed gas after being treated in said hollow cathode dischargeplasma.